1. Field of the Invention
The present invention relates generally to a method and system for monitoring (or supervising) of optical transmission lines, and more particularly to a method and system for monitoring of optical transmission lines suitable for an optical network system for performing transmission between a first terminal device (e.g., a central office) and a plurality of second terminal devices (e.g., subscriber terminals).
2. Description of the Related Art
Referring to FIG. 1, there is shown a conventional system for monitoring of optical transmission lines. This system includes a first optical fiber transmission line 2, N second optical fiber transmission lines 4(#1) to 4(#N) (N is an integer greater than 1), and a branching section 6 for optically connecting the first optical fiber transmission line 2 and the second optical fiber transmission lines 4(#1) to 4(#N). The branching section 6 is provided by an optical star coupler, for example.
A first terminal device 8 is optically connected to an open end of the first optical fiber transmission line 2, i.e., an end of the first optical fiber transmission line 2 opposite to the branching section 6. N second terminal devices 10(#1) to 10(#N) are optically connected to open ends of the second optical fiber transmission lines 4(#1) to 4(#N), respectively. For example, the first terminal device 8 is a central office in an optical fiber network system, and the second terminal devices 10(#1) to 10(#N) are subscriber terminals in the optical fiber network system. Between the first terminal device 8 and each of the second terminal devices 10(#1) to 10(#N), one-way or two-way communication or transmission using an optical signal having a wavelength .lambda.1 is carried out.
N monitor devices (or supervisory devices) 14(#1) to 14(#N) are provided in a monitor area (or supervisory area) 12, so as to detect an abnormality such as a break in each of the second optical fiber transmission lines 4(#1) to 4(#N). The monitor devices 14(#1) to 14(#N) are optically connected to the second optical fiber transmission lines 4(#1) to 4(#N) by WDM (wavelength division multiplexing) couplers 16, respectively. The monitor devices 14(#1) to 14(#N) perform OTDR (optical time domain reflectometry), for example, using monitor light (or supervisory light) having a wavelength .lambda.2 to locate abnormal points in the second optical fiber transmission lines 4(#1) to 4(#N), respectively, or to measure loss characteristics of the second optical fiber transmission lines 4(#1) to 4(#N), respectively.
In the conventional system shown in FIG. 1, the branching section 6 is included in the monitor area 12 where the monitor devices 14(#1) to 14(#N) are provided. Accordingly, many optical fibers are present in the monitor area 12, so that the wiring of the optical fibers in the monitor area 12 is complicated. Further, the plural monitor devices 14(#1) to 14(#N) respectively corresponding to the second optical fiber transmission lines 4(#1) to 4(#N) are required.